Optical Time Domain Reflectometry (OTDR), and more particularly Coherent-OTDR (C-OTDR, e.g. having an OTDR signal based on a continuous wave shifted in frequency and received coherently), is a well known technique for monitoring optical communication link made up of optical fiber spans, in particular for monitoring amplified optical communication link wherein a number of optical amplifiers are cascaded along the optical communication link. For example, in submarine optical communication systems which may span over thousands of kilometers (up to 8-10.000 km) without opto-electronic (O/E) conversion of the WDM signal thanks to a cascade of various tens of WDM amplifiers regularly spaced, C-OTDR allows fast and accurate detection of optical link and/or optical amplifier failures. OTDR relies on the well know principle of the back-scattering of an optical radiation propagating along an optical fiber by the material of the optical fiber itself. Since the optical amplifiers allow propagation only in the direction of the WDM signal along an optical line (e.g. they typically include uni-directional optical isolator), it is needed to route the back-scattered OTDR radiation into a further optical line, co-spanning with the optical line under test, having opposite direction of propagation of the respective WDM signal. This task is usually accomplished by way of a by-pass path in correspondence of each optical amplifier. The Applicant has noted that a general problem of the state of the art OTDR techniques is that, in particular for long optically amplified system (e.g. over 2000 km without O/E conversion), due to the high loss of the C-OTDR bypass path (usually set at about 30 dB), needed for avoiding interference between data (WDM) channels propagating in opposite directions, it is necessary to launch OTDR signals with high power (typically higher than the power of the WDM channels) in order to monitor the integrity and the operation of the amplifiers and/or the lines. This in turns requires to perform the OTDR monitoring operations only out-of-service (i.e. with the WDM signal off), since otherwise the optical power in the OTDR signal would reduce the amplifier optical gain and then the dynamic range of the transmission system. US2009/0324249 A1 discloses various high loss loop back (HLLB) repeater architectures that enable selectively monitoring of Rayleigh signals from both inbound and outbound directions of an optical communication system. In one such embodiment (as shown in FIG. 3a of the cited document), the repeater includes an amplifier pair (amplifiers A and B), six optical couplers, and two wavelength selective filters each reflecting only the two test signal wavelengths from OTDR test equipment. A first HLLB path is provided for coupling the output of amplifier A to the input of amplifier B. A second HLLB path is provided for coupling the output of amplifier B to the input of amplifier A. A third HLLB path is provided for coupling the output of amplifier A to the output of amplifier B. When monitoring the incoming fiber, the two test signal wavelengths from OTDR equipment propagate through the first HLLB path and down the incoming fiber, and the corresponding reflected Rayleigh signal wavelengths from the incoming fiber are provided back to the OTDR test equipment for analysis. In addition to the Rayleigh signal it is also provided an HLLB test signal which is reflected by the filter along the first HLLB path and into port Y of coupler 6. This HLLB test signal is output at port A of coupler 6 and provided to the 10% port of coupler 2, and to the third HLLB path, and then back to OTDR equipment. This HLLB test signal can be used in a similar fashion as the OTDR test signals in diagnosing problems or potential problems associated with the HLLB architecture. The Applicant has noted that the total loss of the third HLLB path (which serves as a by-pass path) is only about 20 dB, resulting in an unacceptably high interference of the in-band back-scattered WDM signals coming from one optical line to the WDM signal of the other optical line, especially when a large number of optically amplified spans are cascaded together. The Applicant has also realized that, in an attempt to overcome the above interference problem, increasing the loss of the third HLLB path (e.g. up to 30 dB) would lead to the disadvantages noted above (need for high power OTDR signals).